In the research and development of medicine and chemicals, storage tubes are extensively used in storing a large number of samples. For example, scientists prepare a large number of samples for a comparative experiment with slightly changing conditions such as blending rate, and they use storage tubes in storing the samples for a required period of time while handling them.
In order to control and store a large number of storage tubes at a time as described above, it is necessary to identify each storage tube. In the prior art, they were identified by handwriting a sample name or identification number directly to the outer surface of storage tubes; however, in recent years sample storages that are controlled by printing a barcode or two-dimensional code on the side and/or bottom surface of the storage tube are highlighted, in which various data and/or control information of a sample are encoded, and then the barcode or two-dimensional code is read in a control process.
Here, the data contents written in the label are a variety of data such as a storing date, a name of the contents, a name of the owner and a serial number written by the identified code such as characters, barcode or two-dimensional code. Regarding the characters, these data can be written by hand-writing, stamp or print. However, barcode or two-dimensional code should be written by printing method.
As mentioned above, it is necessary to print the identification code on the label to identify the individual sample storage body. Especially, the printing technology for the identification code should keep higher contrast ratio even when the printing area is affected by the color of the sample stored in the sample storage body or the surrounding environment in order to read the identification code quickly and accurately.
Recently, the importance of the laser marking technology is increased because it can print the code on label with a non-contact method, high speed and wide applicability. Because of non-contact method, laser marking can print the identification code easily and quickly on the object whose surface has curvature or distortion.
Laser marking is conducted by irradiating the laser beam on the surface of the target object. In this process, the target portion may be overheated locally, and the marking is obtained by physical change of the material or resin on the surface layer, such as to micro-melt, micro-vaporize or micro-burn.
Laser marking method utilizes the change of the physical characteristic or the chemical characteristic of the color former material by absorbing the laser beam energy. Especially, the white laser marking technology which changes black color to white color is useful for printing the identification code because the contrast ratio becomes high.
In the white laser marking technology in the prior art, the white color former is utilized. The white color former is coated on the color forming layer to be changed in color by absorbing the laser beam energy and changing its physical characteristic or its chemical characteristic. The component present in the color former originally exhibits black. The portion that receives laser beam energy is foamed locally. The foams scatter the light, so the foamed portion shows white. For example, a plastic molded object made from thermoplastic resin mixed with additives such as carbon black or titanium sub-oxide originally exhibits black. When it receives laser beam, white color characters, white pattern and white mark can be written on the surface of the plastic molded object (referring to the prior document 1).
Therefore, white color laser marking technology in the prior art requires black color based material for the writable media material. The contrast ratio becomes large if the writable media material exhibits dark color turning to white.
Prior art document 1: JP2006-305926.